The present invention relates to an automatic rangefinder of the zone monitoring type for finding the distance of a target object from a reference plane and, more particularly, to a rangefinder of the type described above for use in a photographic camera for measuring the distance between a zone in which a target object to be photographed lies and the camera.
There is known one zone monitoring type automatic rangefinder for a photographic camera which comprises a light emitter for developing and projecting through a convergent lens a beam of light, pulsated at a predetermined frequency, to illuminate a target object located within one of a plurality of zones at different distances away from the photographic camera, a light receiver including photoresponsive elements substantially equal in number to the number of the zones for detecting the pulsated beam of light illuminating the target object through another convergent lens, said photoresponsive elements being so arranged as to monitor the respective distances of the zones, each of said photoresponsive elements having a parameter the magnitude of which varies as a function of the intensity of the pulsated beam of light received thereby, and means coupled to the light receiver and responsive to change in the magnitude of the parameter of any one of the photoresponsive elements to indicate which range of distances the target object occupies, that is, in which one of the plurality of zones the target object lies.
Examples of the zone monitoring type automatic rangefinder are disclosed, for example, in the U.S. Pat. No. 27,461, reissued on Aug. 15, 1972, and the U.S. Pat. No. 3,723,003, patented on Mar. 27, 1973.
The concept of another zone monitoring type rangefinder is generally based on the principle of triangulation in which a beam of light is projected by a light emitter through a projecting convergent lens and subsequently reflected from a target object and is focused by a receiving convergent lens on one of a plurality of photoresponsive elements of a light receiver located in a predetermined position offset laterally relative to the light emitter so as to determine the altitude of the triangle formed by the light emitter, the target object and the light receiver as a function of the angle of incidence of the reflected light on the receiving convergent lens, this altitude being representative of the distance between the light emitter and the target object.
In this known rangefinder of the type described above, the light emitter and the light receiver are employed separately and positioned, for example, one on each side of the objective lens assembly of the photographic camera and, therefore, a relatively large base length, i.e., the distance between the light emitter and the light receiver, is required so that the rangefinder will have a relatively high resolution, i.e. will measure the distance rather closely. In general, the larger the base length, the higher the resolution of the rangefinder.
However, considering the limited space available in a photographic camera for accommodating the rangefinder, the increased base length does not accord with the recent trend in the photographic industry to manufacture photographic cameras as compact and as light-weight as possible and, therefore, is not feasible. This will now be discussed in connection with another prior art rangefinder disclosed in Japanese Laid-open Patent Publication No. 49-49625, laid open to public inspection on May 14, 1974, and reproduced in FIG. 1 of the accompanying drawings.
Referring now to FIG. 1, the rangefinder disclosed in the above mentioned publication comprises a light emitter 10 capable of producing a flash of light in synchronism with the shutter release in a photographic camera 11 to illuminate a target object T, an image forming lens 12 positioned on one side of the objective lens assembly 11a of the photographic camera 11 remote from the light emitter 10, and a light receiver 13 positioned behind the image forming lens 12 in terms of the direction towards the target object T. The light receiver 13 is constituted by a plurality of photoresponsive elements 14, such as photodiodes or photoconductors, supported in side-by-side relation to each other on a support base 13a and assigned to monitor respective distances of zones away from the camera 11. The image forming lens 12 serves to form an image of the area of the target object T, which has been illuminated by the flash of light from the light emitter 10, of any one of the photoresponsive elements depending on the distances of the zone in which the target object T lies.
In the known rangefinder having the construction shown in FIG. 1, the base length, that is, the distance between the light emitter 10 and the light receiver 13 is relatively large and substantially larger than the maximum outer diameter of the objective lens assembly 11a employed in the camera 11. The reason for the employment of the increased base length for the distance measurement will be described below. In any event, the above mentioned publication also discloses the use of an electro-mechanical arrangement for effecting a focus adjustment of the objective lens assembly 11a in response to the measurement of the altitude of the triangle formed by the light emitter 10, the target object T and the light receiver 12. In particular, the mechanism necessary for driving a focusing lens group mounted within the objective lens assembly 11a is constituted by a return biasing spring, applying a biasing force to the focusing lens group integrally coupled with or linked with a ratchet so that the focusing lens group can linearly move from an infinity or hyperfocal position towards a "near" position, and an electromagnet adapted to selectively engage a pivotally supported pawl member with and disengage it from the ratchet. This pawl and ratchet mechanism is also disclosed in the above mentioned U.S. Reissue Patent, except for the difference in that the ratchet employed in the latter is in the form of a toothed disc so that the focusing lens group can be axially moved while revolving about the optical axis of the focusing lens group.
In general, the resolution of the rangefinder will primarily depend on the base length between the light emitter and the light receiver and the extent to which a confined bundle of rays of light projected from the light emitter 10 spreads as it travels a substantial distance away from the position of the light emitter, which factors are correlated with each other. For example, if the base length is relatively large, a relatively high resolution of the rangefinder can be achieved for a given spread of the bundle of rays of light projected from the light emitter since there is substantially no possibility that the light sensed by the light receiver 13 through the image forming lens 12 impinges upon, that is, is detected simultaneously by, two or more adjacent photoresponsive elements. Stated differently, a substantially equal resolution can be achieved in a rangefinder system wherein a relatively large base length is employed with a relatively great spread of the bundle of rays of light projected from the light emitter and in a rangefinder system wherein a relatively small base length is employed with a relatively small spread of the bundle of rays of light projected from the light emitter.
However, considering the limited space available in the recently developed photographic cameras for accommodating a rangefinder, the employment of a relatively small base length, for example, 20 mm., is desirable. Therefore, in order to achieve a relatively high resolution of the rangefinder, the employment of the relatively small base length creates two problems to be solved. One is to minimize the cross sectional area of the bundle of rays of light from the light emitter, that is, to minimize the light emitting area of the light emitter, and the other is to minimize the size of each of the photoresponsive elements. On the other hand, minimization of the light emitting area of the light emitter and the size of each of the photoresponsive elements are likely to reduce the light energies to such an extent that the rangefinder will fail to operate correctly and satisfactorily. Yet, the employment of the relatively small base length will require a precise positioning of the light emitter and the light receiver relative to each other.
In addition, irrespective of the base length, the employment of the separate light emitter and light receiver such as in any one of the prior art rangefinders require a precise and, therefore, complicated procedure to install the light emitter and the light receiver in a photographic camera at predetermined positions relative to each other. Moreover, the installation of the light emitter and the light receiver is time-consuming and, if done quickly, requires at least two attendant workers resulting in an increased cost of manufacture.